Two rules of thumb:
(1) Do not think of them as ordinal rankings, even rough ordinal rankings: think of them as a list of programs that are worth investigating. You should certainly look at the rounded mean, the median, and the modes, but I wouldn't treat differences between groupings as particularly significant, unless it appears there is a robust consensus (e.g., Group 1 programs have a rounded mean of 5, no Group 2 programs [rounded mean of 4.5] have medians or modes suggesting peculiar differences of opinion).
(2) Ordinarily a student would ask one or two teachers, "Which programs are good in philosophy of X?" The specialty rankings give you the benefit of the answer of seven or ten or twenty professional philosophers. When your teachers disagree with a specialty ranking, ask them to explain why. Remember, too, that the PGR lists the names of evaluators in each specialty. That can aid interpretation of results, if your teachers are knowledgeable.
On social media, after the rankings come out, there's always posts of the form, "The X [where X is a specialty] rankings are bizarre," the translation of which is almost always, "The X rankings don't conform to my individual ranking of the programs," but obviously they conform to the aggregate ranking. Again,the names of evaluators are listed. Unless there's a reason to think the evaluators were not especially good, the right conclusion is not that "X rankings are bizarre," but that opinions of quality differ.
Sometimes it's not just that opinions of quality differ but opinions about what is important in a specialty differ. I'll use an area I know very well to illustrate: "Philosophy of Law." Overall, I think the results are fine, and while this would not be my ranking, I certainly don't think they're bizarre. So, for example, I would not put NYU and UCLA in a Group by themselves; I would not put Toronto in Group 2, but in Group 3 or 4. Penn should be in Group 2. And so on. Of course, I think general jurisprudence is the core of the subject (as it is!), and that without strength in general jurisprudence, a program can't be first-rate in philosophy of law. (Thus, e.g., Toronto is by my lights very weak in general jurisprudence, while strong in some other, but less important, parts of legal philosophy in my judgment.) By contrast, it is absolutely clear to me that some evaluators treated strength in normative jurisprudence as more important than strength in general jurisprudence. The evaluator pool was reasonably balanced between folks with these views about the relative importance of general and normative jurisprudence, so the overall results are reasonable and fair. (As an aside, the fact that any evaluators gave National University of Singapore a 1 in this category tells me they are either very pro-general jurisprudence or limited in their knowledge of the field: very few faculty have two senior figures in two different areas of normative jurisprudence [James Penner in private law theory, esp. philosophy of property; A.P. Simester in criminal law theory].)
A new PGR always elicits by now predictable falsehoods from career PGR haters. One that gave me a very good laugh was a Carnegie-Mellon professor who declared on social media that "overall rankings" are "completely useless" because "philosophy is an extremely diverse field." It's a "joke," said this formal philosopher, to think there is a "principled way to judge X's strength in philosophy of physics against Y's strength in German idealism." Abolish overall rankings, declared this wise, impartial person, with no self-serving prejudices or grievances. [A side note: German Idealism is not a category.]
Now CMU, for those not familiar, is not a philosophy department in any recognizable sense--for those who might expect a philosophy department to have specialists in Plato or Kant or Nietzsche, for example, CMU will be unrecognizable. They don't do what almost all the top-ranked departments do: history of philosophy, and value theory [except applied stuff], and philosophy of language and mind--except the bits of the latter that coincide with what they really do, and do very well, namely, formal philosophy, especially logic, philosophy of mathematics, decision theory and the like. The PGR gives CMU high marks in the latter specialties, but an overall ranking of only #35. Obviously what is happening is that most evaluators do, indeed, have a principled view about what makes a philosophy department a good department, and CMU is so plainly deficient that it doesn't get good marks overall (indeed, I would expect it to come out lower than #35, except that other formal philosophers highly respect what they do, and they are among the evaluators as well). That a CMU professor would prefer that "overall rankings" not exist is understandable, but not a criticism of the PGR. Fortunately, the lack of self-awareness of PGR haters is the gift that keeps on giving every three years.
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